Eukaryotic Cell

Question 1

The cell theory, originally composed in 1838, includes three primary tenets. Name them.

Answer 1

1. The cell is the basic unit of life.
2. All living things are composed of cells, whether one or many.
3. All cells arise from other cells.

Question 2

All cells can be categorized into which two broad groups?

Answer 2

Prokaryotic and eukaryotic cells

Prokaryotic cells are generally simpler and include bacterial species. Eukaryotes can range from single-celled organisms (like yeast) to complex animals (like humans).

Question 3

What main features characterize eukaryotic cells?

Answer 3

Eukaryotic cells have membrane-bound organelles, including nuclei, and linear chromosomes. They are also larger than prokaryotic cells and differ in specific aspects like flagellum structure.

Eukaryotic cells can comprise either unicellular or multicellular organisms.

Question 4

What main features characterize prokaryotic cells?

Answer 4

Prokaryotic cells lack membrane-bound organelles. They generally contain one circular chromosome within a nucleoid region, but can also possess circular plasmids outside the genome.

Prokaryotic cells always comprise unicellular organisms.

Question 5

Define:

cytosol

Answer 5

The cytosol is the fluid contained within a cell.

In contrast, the cytoplasm includes both the intracellular fluid and all of the extranuclear organelles.

Question 6

Define:

cytoplasm

Answer 6

The cytoplasm includes both the intracellular fluid, or cytosol, and the organelles.

The only organelle that is not included in the cytoplasm is the nucleus.

Question 7

Define:

organelle

Answer 7

An organelle is a separate, specialized structure within a cell.

Many organelles are enclosed by lipid bilayers, but some, including ribosomes, are not membrane-bound.

Question 8

Which organelles are membrane-bound?

Answer 8

The nucleus, mitochondria, Golgi apparatus, endoplasmic reticulum, peroxisomes, and lysosomes are membrane-bound. In plants, chloroplasts and vacuoles fall into this category as well.

Ribosomes, on the other hand, are not membrane-bound; while some are attached to the ER, they are not surrounded by membranes of their own.

Question 9

Which organelles contain DNA?

Answer 9

The nucleus is the location of most genetic material, but the mitochondria also contain DNA.

In plants, chloroplasts have DNA as well.

Question 10

What structural features are present in the nucleus?

Answer 10

The nucleus is encased in a double membrane, known as the nuclear envelope. This membrane is marked by channels called nuclear pores. Inside, a fluid (the nucleoplasm) surrounds linear chromosomes.

Question 11

Which genetic processes take place in the nucleus?

Answer 11

The main processes that take place in the nucleus are replication and transcription.

In contrast, translation occurs at ribosomes outside the nucleus. These organelles can either be bound to the ER or located in the cytoplasm.

Question 12

In what part of the cell is the nucleolus located, and what function does it serve?

Answer 12

The nucleolus is located within the nucleus. It serves as the site of ribosomal RNA transcription and synthesis of ribosomal subunits.

Question 13

What membranes and spaces are present in a mitochondrion?

Answer 13

Mitochondria contain both an outer and an inner membrane. The intermembrane space is located between the two, while the mitochondrial matrix is the innermost space, bounded by the inner membrane.

Both membranes are phospholipid bilayers.

Question 14

What main cellular function is performed by the mitochondria?

Answer 14

Mitochondria are involved in cellular metabolism, specifically the production of energy via aerobic respiration.

In the mitochondria, the Krebs cycle produces electron carriers, while the electron transport chain facilitates the formation of a proton gradient. This gradient is used to produce ATP.

Question 15

Binary fission is most relevant to the production of which organelle?

Answer 15

Binary fission is the method of replication for mitochondria.

Just like prokaryotic asexual reproduction, which produces identical daughter cells, this method of division yields identical organelles.

Question 16

What characteristic of mitochondrial inheritance deviates from Mendel’s laws?

Answer 16

Mitochondria are only passed down through the maternal line.

Though sperm contain paternal mitochondria, they are quickly destroyed by the egg after fertilization.

Question 17

A woman has a muscle disease and passes it down to her two sons and three daughters. Her brother has the same disease, but none of his children do. Where is the gene for this condition likely located?

Answer 17

The gene is likely mitochondrial.

Unlike nuclear genes, mitochondrial genes are inherited only through the maternal line. A mother can pass the gene on to both sons and daughters.

Question 18

Which organelle may have arisen as a result of mutualism?

Answer 18

Mitochondria may have evolved from a symbiotic relationship between small bacteria and larger cells. This is known as the endosymbiotic theory.

Like all instances of mutualism, this situation is thought to have provided benefits to both organisms. The smaller bacterium was given a livable environment while providing energy for the larger host.

Question 19

Which cell types are likely to contain an above-average number of mitochondria?

Answer 19

Muscle cells contain large numbers of mitochondria.

Specifically, red or “slow-twitch” muscle fibers are high in mitochondria due to their increased need to perform aerobic respiration. This produces the ATP needed as part of the contraction cycle.

Question 20

What is the cellular role of lysosomes?

Answer 20

Lysosomes break down engulfed pathogens, nutrient molecules, and components of the cell itself that are no longer functional.

Like the stomach, a lysosome contains enzymes and an acidic interior.

Question 21

Describe the structural characteristics of the endoplasmic reticulum (ER).

Answer 21

The ER is a folded membrane divided into two regions: rough ER and smooth ER.

Rough ER contains ribosomes bound to its surface, while the smooth ER does not.

Question 22

Within the cell, what is the role of the endoplasmic reticulum (ER)?

Answer 22

The ER is involved in a variety of processes, with smooth ER and rough ER performing different functions. The smooth ER is involved in lipid anabolism and detoxification, while the rough ER, with its many ribosomes, is the site of protein translation.

Both types of ER help synthesize macromolecules and shuttle them to the Golgi apparatus to be secreted from the cell.

Question 23

What biological products are synthesized in the rough endoplasmic reticulum?

Answer 23

The rough ER synthesizes proteins.

These can include enzymes and peptide hormones, among other examples.

Question 24

What biological products are synthesized in the smooth endoplasmic reticulum?

Answer 24

The smooth ER synthesizes lipids.

These include steroid hormones and phospholipids, among other examples.

Question 25

What is the cellular role of the **Golgi apparatus**?

Answer 25

The **Golgi apparatus** modifies molecules that arrive from the ER. It has the ability to break off into vesicles, and can thus facilitate the exocytosis of these modified products.

Question 26

What are **peroxisomes**, and what cellular function do they perform?

Answer 26

**Peroxisomes** are small membrane-bound organelles that contain enzymes. They function in fatty acid breakdown, detoxification, and facilitation of the pentose phosphate pathway. ## Footnote Peroxisomes are named for hydrogen peroxide (H₂O₂), which can be both formed and broken down within the organelle. This is important because H₂O₂ is a poisonous radical initiator.

Question 27

Which two membrane-bound organelles are unique to plant cells?

Answer 27

Plant cells, unlike animal cells, contain **chloroplasts** and **vacuoles**. ## Footnote Chloroplasts resemble mitochondria and perform the reactions of photosynthesis. Vacuoles are involved in storage.

Question 28

Which eukaryotic cell types are encased in a cell wall?

Answer 28

Fungal and plant cells include cell walls, while animal cells do not. Bacterial cells, though not eukaryotic, also have cell walls. ## Footnote Fungal walls are made of chitin, plant walls are made of cellulose, and bacterial cell walls are composed of peptidoglycan.

Question 29

A cell taken from a multicellular eukaryote lacks chloroplasts but does contain a cell wall. Which type of cell is this?

Answer 29

This is a **fungal** cell. ## Footnote Plants, animals, and fungi are all multicellular, but plants do contain chloroplasts while animals lack cell walls. This sample, then, must come from a fungus.

Question 30

**cytoskeleton**

Answer 30

The **cytoskeleton** is a structural web of protein that can change shape to perform a variety of cellular processes. ## Footnote The prefix "cyto-" means "cell," so the cytoskeleton is the "cell's skeleton."

Question 31

What main functions are executed by the **cytoskeleton**?

Answer 31

The **cytoskeleton** mainly functions to provide shape to the cell, but also is involved in cell movement, endocytosis, and the formation of the spindle apparatus during cell division. ## Footnote Cytoskeletal proteins have the unusual ability to self-assemble into larger units, and can thus change shape based on the cell's needs.

Question 32

Name the three primary, protein-based components of the cytoskeleton.

Answer 32

The three components of the cytoskeleton are: 1. microtubules 2. intermediate filaments 3. microfilaments

Question 33

Describe the structure and role of **microfilaments**.

Answer 33

**Microfilaments** are linear, thin filaments composed of many actin monomers. Microfilaments are thinner than either microtubules or intermediate filaments. ## Footnote As actin polymers, microfilaments play a crucial role in muscle contraction and also facilitate cytokinesis.

Question 34

Describe the structure and role of **intermediate filaments**.

Answer 34

**Intermediate filaments** are fibers composed of various cell-specific proteins; many are composed of keratin. Intermediate filaments are thicker than microfilaments but thinner than microtubules. ## Footnote These fibers contribute to the rigidity of the cell and also form desmosomes, a type of cellular junction.

Question 35

Describe the structural role of **microtubules**.

Answer 35

**Microtubules** are thick, hollow tubulin polymers. Specifically, units of alpha- and beta-tubulin dimerize, and many of these dimers come together to form a microtubule. ## Footnote Microtubules form the spindle apparatus, an essential component of cell division. They also comprise cilia and flagella and perform various other functions.

Question 36

What structural and functional differences exist between **cilia** and **flagella**?

Answer 36

**Cilia** are short, found in large numbers, and beat rhythmically in a back-and-forth pattern. **Flagella** are longer, with only a few found per cell at the most, and beat more slowly but powerfully to propel the cell in a certain direction. In eukaryotes, both cilia and flagella have a 9+2 method of organization. This consists of 9 microtubule pairs arranged in a circle surrounding 2 single microtubules.

Question 37

Which type of cytoskeletal fiber is associated with **MTOCs**?

Answer 37

**Microtubules** are anchored at MTOCs, or microtubule-organizing centers. The fibers attach to the MTOC at one end and radiate toward other parts of the cell. ## Footnote Centrosomes are a specific type of MTOC involved in cell division. Spindle fibers attach to the centrosomes, each of which is composed of two tubulin-based centrioles.

Question 38

Label the following structures on the diagram below: centrosome, centriole, spindle fiber, aster.

Answer 38

Question 39

What factor plays the biggest role in limiting the size of a cell?

Answer 39

The cell must have a sufficiently large **surface area-to-volume ratio**. As the cell becomes larger, volume increases faster than surface area, causing this ratio to decrease. ## Footnote A large surface area gives the cell a greater ability to obtain nutrients and eliminate waste through its cell membrane.

Question 40

What is the cellular role of the **plasma membrane**?

Answer 40

The **plasma membrane**, also called the cell membrane, protects the interior of the cell from its environment. It also limits the movement of specific materials into and out of the cell.

Question 41

Describe the composition of the **plasma membrane**.

Answer 41

The plasma membrane consists of a **phospholipid bilayer**, with polar heads on the exterior (pointing toward the extracellular fluid and cytoplasm) and nonpolar tails on the interior. ## Footnote The membrane also contains cholesterol, glycoproteins, and sphingolipids, among other components. Large proteins are associated with it, both integral (embedded within the membrane) and peripheral (bound to the outside of the membrane).

Question 42

A certain plasma membrane protein is high in valine, isoleucine, and glycine, and contains very few aspartate or lysine residues. What type of protein is this likely to be?

Answer 42

This is probably a **membrane-embedded protein**. ## Footnote Peripheral proteins are located on the exterior of the membrane and are likely to be hydrophilic / charged, while membrane-embedded proteins are positioned inside the bilayer and are generally hydrophobic / uncharged. Valine, isoleucine, and glycine are nonpolar (hydrophobic) amino acids.

Question 43

Explain the **fluid mosaic model**.

Answer 43

The **fluid mosaic model** is used to describe the plasma membrane. It is composed of lipids with a "mosaic" of embedded proteins and other components, and its "fluidity" allows these macromolecule components to move laterally within the membrane.

Question 44

What traits distinguish molecules that can easily pass through the plasma membrane?

Answer 44

Molecules can easily travel through the membrane if they are **small** and **nonpolar**. To move passively, they also must be traveling down their concentration gradient. ## Footnote Large molecules and ions must enter cells through special protein channels or via endocytosis.

Question 45

What is a **concentration gradient**, and what is its biological significance?

Answer 45

A **concentration gradient** is a difference in the amount of solute molecules per unit volume between one region and another. ## Footnote Molecules are prone to moving down their concentration gradient. For example, if the outside of a cell contains a much higher concentration of glucose than the inside, glucose will tend to move into the cell (if possible). If the membrane is impermeable to glucose, water will tend to move out instead.

Question 46

What is the difference between passive and active transport?

Answer 46

**Passive transport** involves the movement of a substance down its concentration gradient without the use of energy. **Active transport** requires energy, usually in the form of ATP, and moves a substance against its gradient.

Question 47

What is **facilitated diffusion**, and is it an active or passive process?

Answer 47

**Facilitated diffusion** is the movement of a substance down its concentration gradient with the help of transmembrane protein channels. ## Footnote Since facilitated diffusion does not require energy, it is a form of passive transport.

Question 48

What traits distinguish molecules that require membrane channels to enter a cell?

Answer 48

Substances that require membrane channels include large molecules, polar molecules, and ions. ## Footnote When these materials move down their concentration gradient, the process is known as facilitated diffusion. Small, nonpolar molecules do not require membrane channels and can travel via simple diffusion.

Question 49

Which type of hormone requires membrane receptors to act on a target cell?

Answer 49

**Peptide hormones**, as well as the catecholamines **epinephrine** and **norepinephrine**, require membrane receptors. ## Footnote As polar molecules, these hormones cannot pass through the hydrophobic cell membrane.

Question 50

What term describes the passive movement of water or another solvent down its concentration gradient?

Answer 50

This process is called **osmosis**. Water will always move from areas of low solute to areas with a higher solute concentration. ## Footnote Osmosis is generally tested in cases where two compartments are separated by a semipermeable membrane. The membrane allows water, but not solute, to pass through; osmosis is thus required to promote similar solute concentrations on both sides.

Question 51

What is the meaning of the term “isotonic?”

Answer 51

An **isotonic** solution is one with the same solute concentration as a solution to which it is compared. ## Footnote For example, fluids that are administered in an IV should be isotonic with human cells. In other words, they should have the same osmolality, or solute concentration.

Question 52

What is the difference between a hypertonic and a hypotonic solution?

Answer 52

In comparison to a reference solution, a **hypertonic solution** has a greater solute concentration, while a **hypotonic solution** has a smaller solute concentration. ## Footnote For example, say that a certain compartment contains 0.2 M NaCl. A solution of 0.1 M NaCl would be hypotonic, while a solution of 0.5 M NaCl would be hypertonic.

Question 53

If a normal human cell is placed in a container of pure water, what will result?

Answer 53

The cell will **swell**, possibly to the point of lysing (rupturing). ## Footnote Since pure water contains no solute, it is hypotonic in comparison to the cell. Water will travel down its concentration gradient from the exterior of the cell to the interior.

Question 54

The hammerhead shark lives in a tropical ocean environment. Are its cells likely to be hypertonic or hypotonic compared to human cells?

Answer 54

Shark cells are likely **hypertonic** to human cells. ## Footnote To avoid excessive water loss or gain, shark cells must be specialized to be isotonic, or at least close, to their surroundings. Since the ocean is composed of salt water, shark cells likely contain more solute than the cells of land-dwelling species.

Question 55

What will happen to a human cell when it is placed in a hypertonic solution?

Answer 55

Water will exit the cell, causing it to **shrivel**. ## Footnote A hypertonic solution is one that contains a comparatively high amount of solute (salt). Due to osmosis, water will flow from a low-solute to a high-solute environment in an attempt to equalize the solute concentrations.

Question 56

# Define: **endocytosis**

Answer 56

**Endocytosis** is a form of transport in which a material is engulfed by the cell membrane, then enters the cell in a vesicle. Endocytosis requires the expenditure of energy. ## Footnote Phagocytosis and pinocytosis are both subtypes of endocytosis.

Question 57

# Define: **exocytosis**

Answer 57

**Exocytosis** is a form of transport in which a material is packaged into a vesicle which then fuses with the cell membrane. This process, which requires energy, allows the material to be exported from the cell. ## Footnote Waste products and secreted hormones often leave a cell via exocytosis.

Question 58

# Define: **pinocytosis**

Answer 58

**Pinocytosis** is a type of endocytosis. In this process, the cell membrane engulfs the extracellular fluid, as well as the small particles it contains, in a vesicle. ## Footnote Other types of endocytosis include phagocytosis and receptor-mediated endocytosis.

Question 59

Name the three distinct junctions that separate the cells of vertebrates.

Answer 59

The three main types of cell junction to know for the MCAT are: 1. tight junctions 2. desmosomes 3. gap junctions

Question 60

What features characterize **tight junctions**?

Answer 60

**Tight junctions** are composed of multiple proteins, including claudins, and form an impermeable seal between adjacent cells. This prevents fluid and solutes from going "around" the cell to enter a cavity. ## Footnote Tight junctions are found between cells in the same epithelial layer.

Question 61

What features characterize **desmosomes**?

Answer 61

**Desmosomes** are composed of intermediate filaments and are found at localized regions throughout a cell's membrane. They generally attach one epithelial or cell layer to another. ## Footnote Desmosomes are found in epithelial and muscle tissue.

Question 62

What features characterize **gap junctions**?

Answer 62

**Gap junctions** are small channels formed from connexin proteins. They allow small solutes and fluid to pass from one cell to another. ## Footnote While gap junctions are found between most cells, they are generally associated with cells that must communicate or function together, like neurons.

Question 63

Which type of junction is especially important in the heart?

Answer 63

**Gap junctions** ## Footnote The electrical impulse that triggers contraction must travel throughout the entire heart muscle, beginning at the SA node. Gap junctions facilitate the movement of ions, allowing the cells to act together as one unit.

Question 64

Name the four phases of the cell cycle.

Answer 64

The four main phases are G₁, S (synthesis), G₂, and M (mitosis). ## Footnote An additional phase is G₀, in which the cell is not immediately preparing for cell division. G₀ can be either temporary or permanent.

Question 65

Which phases of the cell cycle are part of interphase?

Answer 65

G₁, S, and G₂ are parts of interphase. In these phases, the cell grows, replicates its DNA, and prepares for mitosis. ## Footnote The M phase (mitosis itself) is not a part of interphase. G₀ is generally thought to be distinct when it is permanent (as in neurons, which do not divide), but that is unlikely to be tested on the MCAT.

Question 66

What is the significance of the G₀ phase?

Answer 66

The G₀ phase is a stage of prolonged inactivity. In this phase, the cell is not preparing for mitosis. ## Footnote Mature neurons and cardiac muscle are in a permanent G₀ phase, since they do not undergo mitosis.

Question 67

During which phase of the cell cycle would DNA helicase be especially active?

Answer 67

DNA helicase, as well as the other enzymes involved in DNA replication, would be most active during the **S phase**. ## Footnote In the S (synthesis) phase, DNA is replicated to prepare for cell division.

Question 68

What does the cell assess at the G₁ checkpoint?

Answer 68

The **G₁ checkpoint**, also known as the G₁/S checkpoint or the restriction checkpoint, assesses the DNA to ensure that it is in proper condition to be replicated. ## Footnote Passage through this checkpoint is required to enter the S phase of the cell cycle.

Question 69

What does the cell assess at the G₂ checkpoint?

Answer 69

The **G₂ checkpoint**, also known as the G₂/M checkpoint, makes sure that sufficient cell growth has occurred and that the DNA was replicated properly. ## Footnote Passage through this checkpoint is required to enter the M phase of the cell cycle.

Question 70

What is the difference between **germ cells** and **somatic cells**?

Answer 70

**Germ cells**, including sperm and ova, are haploid and participate in reproduction. They are formed via meiosis. **Somatic cells** are diploid and include the rest of the cells in the body. Somatic cells only undergo mitosis.

Question 71

Name the two types of eukaryotic cell division.

Answer 71

The two types of cell division are **mitosis** and **meiosis**. ## Footnote Mitosis produces somatic cells that are identical to the parent, while meiosis produces germ cells with only half of the parental genetic material.

Question 72

Explain the difference between a **haploid** and a **diploid** cell.

Answer 72

**Haploid cells** have only one full set of chromosomes. In humans, the only haploid cells are germ cells (sperm and eggs) produced by meiosis. **Diploid cells** have two distinct sets of chromosomes. In humans, all somatic cells are diploid and are produced by mitosis.

Question 73

List the four phases of mitosis in the order that they occur in a dividing cell.

Answer 73

The four phases of mitosis are: 1. prophase 2. metaphase 3. anaphase 4. telophase and cytokinesis

Question 74

What features distinguish **homologous chromosomes** from **sister chromatids**?

Answer 74

**Homologous chromosomes** are genetically different; each homologous chromosome has the same locus, but different genes, compared to its partner. Homologous chromosomes separate during meiosis I. **Sister chromatids** are genetically identical, as they result from the replication of a single chromosome. Sister chromatids separate during meiosis II and mitosis.

Question 75

Describe the role of the **centrioles** during cell division.

Answer 75

**Centrioles** are cylinder-shaped microtubular structures, generally found in pairs as part of the centrosome. Centrioles act to organize the spindle apparatus along which the chromosomes align.

Question 76

Describe the role of the **centromere** during cell division.

Answer 76

At the **centromere**, sister chromatids attach in pairs. This region also contains the kinetochore, which binds to a spindle fiber and allows the chromatids to be pulled to opposite poles of the cell. ## Footnote The centromere is the location on the chromosome that appears "pinched."

Question 77

Describe the role of the **kinetochore** during cell division.

Answer 77

The **kinetochore** is a complex arrangement of various proteins that forms on the centromere. Here, spindle fibers bind, allowing sister chromatids to be separated to opposite poles of the cell.

Question 78

Name the microtubule-based structure that separates chromosomes during cell division.

Answer 78

The **spindle apparatus** ## Footnote Spindle fibers radiate from the centrosome, which contains centrioles, and bind at the kinetochore region of the centromere.

Question 79

What cellular events characterize **prophase**?

Answer 79

* Loose chromatin condenses into visible chromosomes. * The nuclear membrane disintegrates. * The nucleolus disappears. * The centrioles and spindle apparatus form.

Question 80

What cellular events characterize **metaphase**?

Answer 80

* At this point, spindle fibers are bound to the kinetochore of each chromosome. * The chromosomes line up along the center of the cell, a region called the metaphase plate.

Question 81

What cellular events characterize **anaphase**?

Answer 81

* Sister chromatids are cleaved apart. * Spindle fibers shorten, causing each sister chromatid to move to the opposite pole as its partner.

Question 82

What cellular events characterize **telophase**?

Answer 82

* The cell elongates. * Each daughter nucleus forms a new nuclear membrane and nucleolus. * The chromosomes start to become less condensed.

Question 83

# Define: **cytokinesis**

Answer 83

**Cytokinesis** is the actual formation of two distinct cells, characterized by the distribution of cytosol and organelles and the pinching of the cell membrane between the two nuclei. ## Footnote Cytokinesis happens simultaneously with the later phases of mitosis and meiosis, but is often grouped with telophase.

Question 84

An individual develops a condition where certain cells undergo mitosis rapidly, with interphase dramatically shortened. What name is commonly used for this condition?

Answer 84

**Cancer** ## Footnote Cancer can be promoted by many factors, including carcinogens, DNA damage, failure of tumor suppressor genes, and failure of apoptosis, or a combination of these events.

Question 85

Name four broad differences between **mitosis** and **meiosis**.

Answer 85

* Mitosis involves a single division, while meiosis involves two. * Mitosis produces two identical diploid cells, while meiosis produces four haploid cells that are not identical. * Mitosis is used to form somatic cells, while meiosis produces germ cells (gametes). * Crossing over occurs in meiosis, but does not happen in mitosis.

Question 86

Mitosis is broadly similar, but not identical, to meiosis I. Name three differences between the two processes.

Answer 86

* Mitosis involves the separation of sister chromatids, while meiosis I involves the separation of homologous chromosomes. * Mitosis produces diploid cells, while the products of meiosis I are haploid. * Crossing over occurs in prophase of meiosis I, but does not occur at all during mitosis.

Question 87

During which stage of meiosis do daughter cells become haploid?

Answer 87

Daughter cells are already haploid after meiosis I. ## Footnote Meiosis I is known as "reductional division," since homologous chromosomes separate during this stage. This leaves two daughter cells, each with a single chromosome from each pair and its replicated copy. Meiosis II, or "equational division," separates these copies.

Question 88

Which two processes ensure that none of the four daughter cells produced during meiosis are genetically identical?

Answer 88

**Crossing over** and **independent assortment** ## Footnote Crossing over is a method of genetic recombination that occurs during prophase I of meiosis. In this process, homologous chromosomes exchange genetic material, increasing total variation. The principle of independent assortment, or Mendel's second law, states that genes for different traits separate into gametes independently of each other (as long as those traits are not linked).